In general, a GNSS system is made up of a plurality of satellites allowing a mobile receiver to determine its position in a land-based plane of reference, its speed and the time.
There are currently several GNSS systems, which in particular include the GPS system, the GLONASS system and the GALILEO system, which is expected to be brought online soon.
The satellites of such a GNSS system are able to emit electromagnetic signals in particular comprising navigation information.
Each item of navigation information generally comprises data relative to the transmission time by the satellite of the corresponding signal and the current position of the satellite. In particular, the data relative to the current position of the satellite generally contain the almanac providing a rough position of the satellite and the ephemerids giving the exact current position of the satellite.
The item of navigation information is carried by a carrier wave and modulated by a spreading code specific to each satellite. Thus, the signals are transmitted by the satellites using a spread spectrum technique.
The receiver is able to receive the signals emitted by the satellites and to extract the navigation information therefrom in particular to determine the distance to the satellite that transmitted the corresponding signal. This distance, also called pseudo-distance, is determined by analyzing the propagation delay of the corresponding signal.
To determine its position, its speed and the time, the receiver performs a digital processing of the navigation information from at least three different satellites.
In practice, to have a more precise position, the receiver needs navigation information from at least four different satellites.
More specifically, to acquire the navigation information from a given satellite, the receiver implements two phases processing the signals from this satellite.
During an initial phase, called acquisition phase in the state of the art, the receiver generates a local signal in particular containing a local spreading code showing the image of the spreading code of the satellite.
Since the receiver does not initially know its position, the local signal is not synchronized with the received signal. This in particular means that the local signal is carrier frequency-shifted from the received signal by a value called Doppler value, and the spreading code of the received signal is delayed from the local spreading code by a value called delay value.
The receiver then searches for a peak in the correlations between the local signal and the received signal by trying different Doppler and delay values.
When a peak is detected, the receiver determines the Doppler and delay values corresponding to this peak and, from these values, launches a following phase, called tracking phase in the state of the art.
During the tracking phase, the receiver regularly updates the Doppler and delay values, and extracts the item of navigation information from the signal transmitted by the satellite in particular using the local spreading code and the determined Doppler and delay values.
At the end of the acquisition phase, the receiver is considered to have synchronized itself with the satellite, or has “locked on” to the satellite.
This in particular means that the receiver was able to find the Doppler and delay values relative to this satellite to initiate the tracking phase.
Sometimes, the receiver synchronizes its local signal corresponding to the desired satellite with the signal received from another satellite, which leads to an erroneous distance measurement, and therefore potentially a wrong position.
In this case, this is a wrong synchronization, or a false “lock”.
This for example occurs when the correlation between the local signal and the signal received from the sought satellite provides less energy than the correlation with the signal received from another satellite, due to a large received power deviation.
Different methods exist in the state of the art making it possible to avoid such a wrong synchronization.
Thus, one method, used conventionally, consists of verifying the consistency between the position of the satellite computed from ephemerids contained in the item of navigation information and that computed from the almanac, containing the identifiers of the satellites, unlike the ephemerids. An inconsistency between these values therefore indicates a wrong synchronization.
However, this method is not completely satisfactory. In particular, it requires collecting all of the ephemerids contained in the navigation information, which is relatively time-consuming. In practice, this may take up to two minutes.
One can then see that this is detrimental to the operation of the receiver, in particular after the satellite is hidden, and does not make it possible to ensure continuous service.